The present invention relates generally to the field of loudspeakers, and more particularly, to a loudspeaker having a dual chamber acoustical enclosure having two external vents and one internal vent.
A common objective in designing loudspeaker systems is to improve acoustical performance in the operating band of the system and to minimize distortion caused by, among other things, loudspeaker cone excursions at frequencies at and below a lower cutoff frequency of the system.
In general, when a loudspeaker is energized, its electro-acoustic transducer diaphragm (“cone”) reciprocates or vibrates at a frequency which varies with the signal input to the loudspeaker. When an unmounted or unbaffled loudspeaker is operated in a so-called “free air” mode, its cone exhibits large mechanical excursions as it approaches its resonant frequency, which produces significant acoustical distortion. In order to control this motion and thereby reduce the distortion level of the loudspeaker, it is customary to mount the loudspeaker in some form of housing or loudspeaker enclosure.
In its simplest form, this enclosure is a closed box with the loudspeaker mounted or suspended in an opening in one wall thereof. Such a loudspeaker system causes the large amplitudes of the loudspeaker cone excursions to occur at a different frequency, thus changing the resonant frequency of the loudspeaker relative to its resonant frequency in its “free air” mode of operation.
U.S. Pat. No. 4,549,631, issued to Amar G. Bose, discloses an acoustic suspension loudspeaker system that has an acoustical enclosure of rectangular cross-section with a baffle dividing the interior of the enclosure into first and second subchambers. The acoustical enclosure of the loudspeaker system disclosed by Bose is commonly referred to as a “bass reflex” enclosure. Each subchamber of this enclosure has a port tube (“vent”) that couples the respective subchamber to the exterior environment outside of the enclosure. The dividing baffle carries a woofer. This type of acoustical enclosure can be thought of as a dual chamber acoustical enclosure having two “external” vents. Each external vent serves as a passive radiating means. More particularly, each external vent provides an acoustic mass that constitutes an extra reactance which can be used to tailor the frequency response of the loudspeaker system at the low end. A ported or vented system is characterized by a resonance (port resonance) at which the mass of air in the port (vent) reacts with the volume of air within the enclosure to create a resonance at which the excursion of the loudspeaker cone is minimized. The dual chamber acoustical enclosure provided with two external vents disclosed by Bose provides improved sensitivity at port resonance which results in an extension of the lower cutoff frequency of the loudspeaker system to a lower value, while also reducing loudspeaker cone excursions in the vicinity of the lower cutoff frequency of the loudspeaker system.
However, for bass reflex loudspeakers of the type disclosed in U.S. Pat. No. 4,549,631 (Bose) to achieve a flat bandpass response, a loudspeaker driver with a rather high magnetic efficiency is required, which is expensive. Moreover, bass reflex loudspeakers which utilize two subchambers having ports for directly acoustically coupling each of the respective subchambers to the exterior environment, tend to provide poor response for acoustic frequencies falling between the resonant frequencies of the two subchambers and their corresponding respective ports when the resonant frequencies of the two subchambers vary by more than a factor of 3 to 1.
U.S. Pat. No. 4,875,546, issued to Palo Krnan, discloses a two-chamber bass reflex type loudspeaker that overcomes the above-noted deficiency of the Bose loudspeaker system. In particular, the Krnan loudspeaker system exhibits good frequency response for frequencies between the resonant frequencies of the two subchambers of the two-chamber enclosure, even when these resonant frequencies are separated by a factor of up to 10 to 1. The Krnan loudspeaker system includes a first subchamber that is pneumatically and acoustically coupled with the second subchamber via a first port (vent) that is sized to enclose a first acoustic mass of air while one of the subchambers is pneumatically and acoustically coupled with the outside environment via a second port (vent) that is sized to enclose a second acoustic mass of air. By properly constructing the first and second subchambers and first and second ports, the acoustical enclosure will operate as an acoustical bandpass filter in which high frequency distortion components such as those generated by diaphragm excursions of the transducer (speaker cone) will be acoustically attenuated.
Although the Krnan loudspeaker system described above does overcomes some of the problems inherent with electrical filtering via crossover networks, and does exhibit better performance over a broader operating band than the Bose loudspeaker system described above, it still has significant drawbacks and shortcomings. More particularly, the efficiency of the Krnan loudspeaker system is less than desirable, and the distortion products generated in the vicinity of the lower cutoff frequency are greater than is desirable.
It should be mentioned that Japanese Published Application Number 4-301998, naming Kinihiko Tamura as the inventor, discloses a dual-chamber loudspeaker system that features a “triple-vented” acoustical enclosure, with two “external” vents that pneumatically and acoustically couple respective subchambers to the exterior environment, and one “internal” vent that pneumatically and acoustically couples the first and second subchambers of the enclosure to one another. This dual-chamber, triple-vented loudspeaker system is a low band (i.e., bass) loudspeaker system. The internal vent is specifically designed and used to minimize distortion due to loudspeaker cone excursions at frequencies lower than the resonant frequency (i.e., it sharpens the upper cutoff frequency of the bass speaker), but does not contribute to acoustical output within the normal operating band. In fact, Tamura teaches that even in the narrow low frequency band of interest in his system, the internal vent actually acts as a bypass circuit whose effect is to reduce the acoustical output from the external vents, as well as to reduce the level of the distortion.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a loudspeaker system having an acoustical enclosure that overcomes the above-discussed drawbacks and shortcomings of the presently available technology. The present invention fulfills this need in the art.